This invention relates to an acceleration detector and more particularly to an acceleration detector which includes an acceleration transducer secured to a base by a screw.
FIGS. 1 and 2 illustrate one example of the conventional acceleration detector disclosed in Japanese U.M. Publication No. 60-23730. In the figures, the conventional acceleration detector comprises a housing 1 made of a conductive material. The housing comprises a base 1a which has at its bottom a thread 1b for securing the housing 1 to an article (not shown), such as an internal combustion engine, of which acceleration or vibration upon knocking is to be detected. The housing 1 also comprises a cylindrical wall 1c defining therein a cavity for receiving therein an acceleration detecting transducer 10. The transducer 10 comprises a pair of ring-shaped piezoelectric elements 2 sandwiching a ring-shaped terminal 3 shown in FIG. 2 therebetween. The terminal 3 is made of a sheet of phosphor bronze and has a ring or an eye portion 3a having a circular opening 3b and a radially outwardly extending tab 3c having a small hole 3d for connecting one end of a lead wire 7 for supplying an electrical signal from the piezoelectric elements 2. The piezoelectric elements 2 and the terminal 3 are placed on the inner bottom surface of the base 1a. A cylindrical inertial weight 4 having a central bore is stacked on the piezoelectric element 2. The transducer assembly of these elements 2, 3 and 4 are secured to the base 1a of the housing 1 by a mounting screw 5. It is seen that a threaded portion 5a of the screw 5 is thread-engaged into a threaded hole 1d of the base 1a and that a head 5b presses the weight 4 against the piezoelectric elements 2. In order to electrically insulate the piezoelectric elements 2 and the terminal 3 from the mounting screw 5, an electrically insulating tube 6 made of polyethylene is placed over the threaded portion 5a of the mounting screw 5. The thickness of the terminal 3 may be 0.1 mm-0.2 mm and the thickness of the insulating tube 6 may be 0.2 mm-0.3 mm. The remaining portion of the cavity defined within the housing 1 is filled with an electrically insulating semi-solid resin 8.
When in use, the acceleration detector is securely mounted on an internal combustion engine (not shown) for example by threaded bolt 1b of the housing 1. The acceleration or the vibration of the internal combustion engine produces the movement of the inertial weight 4 relative to housing 1, which causes the piezoelectric elements 2 to be stressed by the inertial weight 4, whereby an electrical signal indicative of the movement of the inertial weight 4 relative to the engine is generated from the piezoelectric elements 2. The electrical signal is provided through the washer terminal 3 and the lead 7 to be analyzed to determine as to whether or not a knocking signal which is generated upon knocking of the internal combustion engine is present. When it is determined that a knocking signal is contained in the electrical signal, the operating parameters for operating the engine can be adjusted to increase the output power or decrease the fuel consumption rate.
During the assembly of the acceleration detector of the above-construction, it is often occurs that the lead wire 7 is pulled in the direction of an arrow A when the bolt 1b is to be thread engaged into the engine (not shown). Then, the terminal 3 connected to the lead wire 7 is pulled in the radial direction and is displaced relative to the piezoelectric elements 2, causing the insulating tube 6 to be cut between the inner edge of the ring portion 3a of the terminal 3 and the ridges of the thread 5a of the screw 5. When the insulating tube 6 is broken and the terminal 3 is brought into contact with the mounting screw 5, the piezoelectric elements 2 are short-circuited and no output signal is provided.
Also, since the axial length of the threaded portion 5a is unnecessarily long and extends not only within the threaded hole in the base 1a but also over the bolt stem portion and extends completely through the insulating tube 6 and partly through the inertial weight 4, additional time and cost for forming unnecessary thread are needed.